The field of the invention relates to fuel vapor recovery systems. In particular, the invention relates to control of fuel vapor recovery in engines equipped with air/fuel ratio feedback control systems.
Modern engines are equipped with three-way catalytic converters (NO.sub.x, CO, and HC) to minimize emissions. Efficient operation requires that the engine's air/fuel ratio be maintained within an operating window of the catalytic converter. For a typical converter, the desired air/fuel ratio is referred to as stoichiometry which is typically 14.7 lbs. air/1 lb. fuel. During steady state engine operation, the desired air/fuel ratio is achievable by an air/fuel ratio feedback control system responsive to an exhaust gas oxygen sensor. More specifically, a desired fuel charge is first determined by dividing a measurement of inducted airflow by the desired air/fuel ratio. Electronically actuated fuel injectors are actuated in response to the desired fuel charge determination. This desired fuel charge is then trimmed by feedback from a correction factor responsive to the exhaust gas oxygen sensor such that actual engine operation is maintained near the desired air/fuel ratio.
Air/fuel ratio control has been complicated, and in some places made unachievable, by the addition of fuel vapor recovery systems. To reduce emission of gasoline vapors into the atmosphere, as required by federal emission standards, fuel vapor recovery systems are commonly utilized. These systems store fuel vapors emitted from the fuel tank in a canister having activated charcoal or other hydrocarbon absorbing material. During engine operation above a minimum speed and temperature, fuel vapors from both the fuel tank and storage canister are inducted into the engine. Induction of rich fuel vapors creates at least two types of problems for air/fuel ratio control. Since there is a time delay for air/fuel charge to propagate through the engine and exhaust, any perturbation in the air/fuel ratio of the inducted air/fuel charge results in an air/fuel transient. Thus, perturbing the inducted air/fuel charge by introducing purged fuel vapors may cause an air/fuel transient resulting in an emissions increase. Further, conventional air/fuel ratio feedback control systems have a range of authority. Induction of rich fuel vapors may exceed the feedback systems range of authority resulting in an unacceptable increase in emissions.
U.S. Pat. No. 4,715,340 issued to the same inventive entity as herein has addressed some of the above problems. More specifically, a combined air/fuel ratio feedback control system and vapor purge system is disclosed. To reduce the air/fuel transient which may occur during the beginning of a purge cycle, the rate of purge flow is controlled via a solenoid valve such that purge flow rate is ramped on at a slow rate. Further, the purge flow is made proportional to inducted airflow. In general, control of the purge flow is accomplished by duty cycle modulation of the "on time" of the solenoid valve. Stated another way, purge flow is proportional to the pulse width of a valve actuating signal. This pulse width is made proportional to inducted airflow.
The inventors herein have recognized that because the solenoid valve is sized for maximum purge flow, the valve is nonlinear, and may not turn on at all, at low purge flow rates. For example, at narrow pulse widths corresponding to low inducted airflow, the solenoid valve may not be actuated for a sufficient period of time to turn on. Thus, over the desired operating range, purge flow may not be maintained as a linear proportion of airflow, thereby causing an undesired air/fuel transient. This nonlinearity is becoming exacerbated with the increasing need to purge fuel vapors in view of tightening federal and state evaporative emission standards. It is becoming desirable to increase purge flow rates resulting in larger purge valves and accordingly more nonlinearity at low flow rates. Further, it is also becoming necessary to purge more often such as during light engine loads and idle. Both of these trends result in an exacerbation of the nonlinear disadvantages of prior approaches.